HP All In One 6500 Error: Paper Size Error

We have a houseful of MacBook Pro Laptops and use the HP 6500 printer in a wireless configuration. This printer was a replacement for another HP All In One wireless printer that worked great for almost 2.5+ years. Anyway it's been funcitonal for about 2 months, but after changing out the regular 8.5" X 11" paper with some 4" X 6" photo paper and printing a bunch of photos, we've started having problems. The error that keeps coming up is:

Paper Size Error: The paper size selected in the printer driver software does not match the paper size in the printer IN tray.

Also, the error on the printer screen reads: Paper size or type is incorrect. Press OK to cancel job.

This error will occur in different applications and different type of files and I can't seem to figure out a pattern. For example, it will happen trying to print out a PDF file, sometimes with an Adobe Illustrator file, sometimes with a Word file, etc.

I do a bunch of different things like turn off the printer, let it rest a few minutes and turn it back on. Reboot my laptop. Uninstalled all the printer software and reinstalled it. Sometimes it takes all these details to get everything back working again and sometimes I have to do them again and again.

Also, if I pull up another file, they will often print. (or after I delete the queded bad file).

If I take my laptop and hook it up to a inexpensive HP 1600 printer we also own, I can print all the same files that are causing the error to happen on the HP 6500.

Help! I've looked all over the internet for anyone else that might be having this problem to no avail.

I DO NOT want to call HP customer NO service. There has to be something simple for a solution.